Is it possible to be fair without being reasonable? Or vice versa?
It for sure is possible to be fair without being reasonable. Sometimes one would do things that seem dumb, against all reason, yet are fair in the sense of fair play.
And many times people decide to be reasonable, even if it is not fair to somebody else.
And of course there are the many other uses of fair that cannot be replaced at all with reasonable:
and many more. Have a look at the word reasonable too for comparison.